The invention relates to a device in a paper machine for guiding a web and in particular for guiding a leader of the web. The device is arranged in a paper machine in an area between a center roll, or equivalent, of the press section and a separate press nip in the press section. The separate press nip is preferably formed between a smooth-faced upper roll and a hollow-faced lower roll. A lower felt is arranged to run around the lower roll and from a guide roll, preferably a suction-transfer roll, placed in proximity to the center roll, as a straight run into and through the separate press nip.
The invention also relates to a method for guiding a web and particularly a leader of the web and an arrangement of a press section in which the device for guiding the web and/or leader is employed.
When a paper machine is started, or after a web break has occurred, an end of the web is passed to the dryer section by cutting, a narrow leader or lateral band which typically has a width of about 200 mm, for example by means of a water jet. In prior art devices, the leader is usually guided manually by means of air jets. However, the constantly increasing running speeds of paper machines has resulted in increasing difficulties in the threading of the web. These difficulties are most severe when a separate press is employed inside the press section, immediately after the press section in the initial part of the dryer, and in gaps between the groups of drying cylinders in the drying section.
In prior art devices, press sections are used that comprise a compact combination of press rolls consisting of two or three press nips placed closed to each other. The web has a closed draw between these press nips. After the combination of rolls, a third or fourth separate press nip follows which is formed between a smooth-faced upper roll and a hollow-faced lower roll. A lower felt is passed through this separate nip. The hollow-faced lower press roll is placed inside a loop of the lower felt. The web of full width and the leader are guided from the smooth-faced center roll of the compact combination of rolls as a short free draw usually over a guide roll onto the upper face of the felt in the separate nip.
On the upper roll of the separate press nip, a broke conveyor and a doctor are commonly employed. The doctor is arranged such that the lateral band (leader) that attempts to be wound around the upper roll is shifted to the side in the transverse direction. The broke conveyor is placed above a substantially horizontal run of the felt of the separate press, which run is guided by a suction-transfer roll into the separate nip. On this run of the web, a suction box is employed immediately before the separate nip to ensure that the web and the leader remain on the top face of the felt. Below the broke conveyor, there is usually a wall which is substantially parallel to the run of the felt that runs underneath the wall. This wall and the straight felt run define a gap therebetween which is a difficult problem point in the threading of the web because of the air flows induced in this gap and the dynamic pressure effects of these air flows.
The lateral band does not adhere to the felt of the separate press after it has been separated from the center roll and blown further. Rather, air remains between the lateral band and the felt. Separation of the web from the felt is also promoted by a strong air current that takes place in the gap defined by the felt and the bottom wall of the broke conveyor placed above the felt. This air current attempts to absorb the web towards the bottom wall of the broke conveyor which causes problems in the operation of the paper machine. These problems have become more serious with increasing running speeds of paper machines.
These problems of threading are also increased by the fact that, in the threading of the leader, the suction zone on the suction roll of the lower felt of the separate press cannot be utilized efficiently in order to made the leader adhere to the lower felt. This is because the suction cannot be applied effectively to the area of the leader as the suction zone "leaks" over the major part of its width in which there is no sealing web. A further problem is constituted by the bag formations arising from the slackness of the leader after the separation from the center roll, and by other, corresponding problems.
In prior art devices, attempts have been made to reduce the slackness of the leader after the center roll in the press by means of a difference in the running speed of the felt of the separate press. However, with increasing running speeds of paper machines, a sufficient reserve for an increase in the running speed in this respect is generally not available.
With respect to the prior art most closely related to the present invention, reference is made to Finnish Patent Nos. FI 69,145 and FI 78,528 (equivalent of U.S. Pat. No. 4,923,567) and to Finnish Patent Application No. FI 915342 (filed on Nov. 12, 1991), in which attempts have been made to solve the problems discussed above.
In FI Pat. Appl. No. 915342, a device in a paper machine similar to that defined above is described for the threading of the leader of the web. In this device, a transverse blow pipe or pipes is/are fitted before the separate press nip on the straight run of the felt of the separate press nip. Blowings or air jets are applied from the blow pipe(s) in a direction opposite to the running direction of the felt and of the web. By means of the blowings, induction of air into the gap space between the straight run of the felt and the wall placed above the run is reduced and, at the same time, the staying and keeping of the leader on the face of the felt on the straight run of the felt before the separate press nip is promoted.